The invention relates to drill chucks of the kind including a body part in which jaws are slidably mounted in respective guides and which generally require very careful and rigidly strict inspection during or immediately following manufacture to ensure that a drill bit gripped between the jaws will rotate with almost perfect truth, that is to say within acceptable limits of inaccuracy. If a drill bit does not rotate with almost perfect truth, it will produce oversize holes and this is a particular problem when drilling holes to receive plastics or fibre wall plugs (especially with a hand held drill) since adequate tightening of the plugs in such holes may not then be possible, hence the need for very careful and rigidly strict inspection of the drill chucks. Such inspection is itself expensive and of course results in some of the finished chucks being rejected so that they must be stripped down again for their component parts to be used in other chucks. (It will be understood that unacceptable inaccuracy wll usually result from a build-up of tolerances on the component parts so that the re-assembly of the component parts in other chucks will frequently cure the trouble. Nevertheless the work involved is expensive and increases the cost of production per chuck sold). The object of the invention is at least to alleviate this present difficulty and to reduce the necessity for inspection as much as possible.
According to the invention, there is provided a drill chuck including a body part in which jaws are slidably mounted in respective guides, a bevel gear element associated with a nut rotatably mounted on the body part and having screwthreaded engagement with the jaws, said bevel gear element being capable of being engaged by the teeth of a chuck key having a pilotend which can be received in bores extending radially of the body part; and mounting means provided on a further part for connecting the chuck to a power driving spindle, means being provided for producing a self-centering action between the body part and said further part when drilling. The means provided for producing a self-centering action between the body part and the further part provided with mounting means for connecting the chuck to a power tool driving spindle may take the form of a steel ball located at the axis of the chuck between the two parts, in seatings formed in said two parts. Alternatively, such means may take the form of a domed portion formed on one of said parts at the axis of the chuck and arranged to engage a seating in the other part. A cylindrical or generally cylindrical sleeve may extend rearwards of the body part and may form at least a part of the further part provided with mounting means for connecting the chuck to a power tool driving spindle. Said sleeve may be provided at one end with the mounting means for connecting the chuck to a power tool driving spindle and be connected at its outer end to the bevel gear element associated with the nut rotatably mounted on the body part and having screwthreaded engagement with the jaws, the arrangement being such that torque applied to the chuck from the power tool driving spindle when drilling tends to rotate the nut relative to the body part to tighten the jaws.